From mboxrd@z Thu Jan 1 00:00:00 1970 From: David Woodhouse Subject: Re: [RFC,05/10] x86/speculation: Add basic IBRS support infrastructure Date: Mon, 29 Jan 2018 21:02:39 +0000 Message-ID: <1517259759.18619.38.camel@infradead.org> References: <1516476182-5153-6-git-send-email-karahmed@amazon.de> <20180129201404.GA1588@localhost.localdomain> <1517257022.18619.30.camel@infradead.org> <20180129204256.GV25150@localhost.localdomain> <31415b7f-9c76-c102-86cd-6bf4e23e3aee@linux.intel.com> Mime-Version: 1.0 Content-Type: multipart/signed; micalg="sha-256"; protocol="application/x-pkcs7-signature"; boundary="=-xYeScXqhiFpOu6UrlUXf" Cc: KarimAllah Ahmed , linux-kernel@vger.kernel.org, Andi Kleen , Andrea Arcangeli , Andy Lutomirski , Ashok Raj , Asit Mallick , Borislav Petkov , Dan Williams , Dave Hansen , Greg Kroah-Hartman , "H . Peter Anvin" , Ingo Molnar , Janakarajan Natarajan , Joerg Roedel , Jun Nakajima , Laura Abbott , Linus Torvalds , Masami Hiramatsu , Paolo Bonzini , To: Arjan van de Ven , Eduardo Habkost Return-path: In-Reply-To: <31415b7f-9c76-c102-86cd-6bf4e23e3aee@linux.intel.com> Sender: linux-kernel-owner@vger.kernel.org List-Id: kvm.vger.kernel.org --=-xYeScXqhiFpOu6UrlUXf Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Mon, 2018-01-29 at 12:44 -0800, Arjan van de Ven wrote: > On 1/29/2018 12:42 PM, Eduardo Habkost wrote: > >=20 > > The question is how the hypervisor could tell that to the guest. > > If Intel doesn't give us a CPUID bit that can be used to tell > > that retpolines are enough, maybe we should use a hypervisor > > CPUID bit for that? > > the objective is to have retpoline be safe everywhere and never use IBRS > (Linus was also pretty clear about that) so I'm confused by your question The question is about all the additional RSB-frobbing and call depth counting and other bits that don't really even exist for Skylake yet in a coherent form. If a guest doesn't have those, because it's running some future kernel where they *are* implemented but not enabled because at *boot* time it discovered it wasn't on Skylake, the question is what happens if that guest is subsequently migrated to a Skylake-class machine. To which the answer is obviously "oops, sucks to be you". So yes, *maybe* we want a way to advertise "you might be migrated to Skylake" if you're booted on a pre-SKL box in a migration pool where such is possible.=C2=A0 That question is a reasonable one, and the answer possibly the same, regardless of whether the plan for Skylake is to use IBRS, or all the hypothetical other extra stuff. --=-xYeScXqhiFpOu6UrlUXf Content-Type: application/x-pkcs7-signature; name="smime.p7s" Content-Disposition: attachment; filename="smime.p7s" Content-Transfer-Encoding: base64 MIAGCSqGSIb3DQEHAqCAMIACAQExDzANBglghkgBZQMEAgEFADCABgkqhkiG9w0BBwEAAKCCEFQw ggUxMIIEGaADAgECAhBNRhEyk/HZ7naOeTHWrzuAMA0GCSqGSIb3DQEBCwUAMIGXMQswCQYDVQQG EwJHQjEbMBkGA1UECBMSR3JlYXRlciBNYW5jaGVzdGVyMRAwDgYDVQQHEwdTYWxmb3JkMRowGAYD VQQKExFDT01PRE8gQ0EgTGltaXRlZDE9MDsGA1UEAxM0Q09NT0RPIFJTQSBDbGllbnQgQXV0aGVu dGljYXRpb24gYW5kIFNlY3VyZSBFbWFpbCBDQTAeFw0xNzEyMjEwMDAwMDBaFw0xODEyMjEyMzU5 NTlaMCQxIjAgBgkqhkiG9w0BCQEWE2R3bXcyQGluZnJhZGVhZC5vcmcwggEiMA0GCSqGSIb3DQEB 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